Add test coverage for aggregation system variables

Summary

Add comprehensive test coverage for aggregation system variables. This includes feature tree entries, smoke tests, and detailed compatibility tests.

Variable	Context	Priority
`$$ROOT`	Full current document	High
`$$CURRENT`	Current document (equivalent to field paths)	High
`$$REMOVE`	Remove field from output	High
`$$NOW`	Current datetime	High
`$$CLUSTER_TIME`	Current cluster time (replica set)	Medium
`$$DESCEND`	`$redact` stage	Medium
`$$PRUNE`	`$redact` stage	Medium
`$$KEEP`	`$redact` stage	Medium
`$$USER_ROLES`	Current user roles	Low

Add feature tree entries under core,operator,expressions,variable,system-variables
Add smoke tests for each supported variable
Add shared expression engine tests under expressions/variable/system-variables/:
- $$ROOT — in $project, $addFields, nested paths ($$ROOT.a.b), inside $cond
- $$CURRENT — equivalence with field paths, in various expression contexts
- $$REMOVE — in $project, $addFields, conditional removal ($cond + $$REMOVE), nested fields
- $$NOW — type verification, consistency within a pipeline, usage in date expressions
- $$DESCEND/$$PRUNE/$$KEEP — with $redact stage
- $$CLUSTER_TIME — type verification, availability

documentdb_tests/compatibility/tests/core/operator/expressions/variable/system-variables/

$let variable scoping is already covered in PR Add $let and $literal tests #102
Per TEST_COVERAGE.md, system variables are shared expression engine behavior — test once with a representative operator, not per-operator

/take

@vic-tsang

Assigned to @vic-tsang!

@eerxuan

confirmed with @eerxuan that we will skip $$DESCEND, $$PRUNE, $$KEEP, and $$USER_ROLES

@vic-tsang